As used herein, a "semiconductor device" is a silicon chip (die) containing circuit elements. A "semiconductor device assembly" is a silicon chip contained within a package and connected (wired or bonded) to conductive leads which exit the package.
Heat is inevitably generated during operation of a semiconductor device, and may become destructive of the device if left unabated. The problem of heat dissipation is especially relevant in semiconductor devices that have a high lead count (e.g., high I/O) or which operate at high speeds, both of which factors contribute significantly to the generation of heat by the device.
It is generally well known to provide some sort of heat sink for semiconductor devices. Heat sinks generally include at least a heat-transferring portion positioned in close proximity to the semiconductor device (die) for efficiently extracting heat therefrom, and a heat-dissipating portion remote from the die with a large surface area for dissipating heat. The heat-dissipating portion is typically formed with a number of parallel fin layers, through which air passes to remove heat from the heat sink. Typically, the entire heat sink structure may simply be disposed on an exterior of a package, such as on the lid of a lidded package.
Commonly-owned U.S. Pat. No. 5,175,612 discloses a heat sink for semiconductor device assembly. As shown for example in FIG. 3 therein, the heat sink has a fixed portion embedded within the semiconductor chip assembly and an "add-on" portion which resides entirely exterior the semiconductor chip assembly. The fixed portion extends to close proximity with the die, and terminates in a flat upper surface forming an exterior surface of the semiconductor chip assembly. The add-on portion, shown as having two fins, is provided with a button which fits into a recess in the upper surface of the fixed portion. This two-part heat sink arrangement exhibits some degree of "flexibility" in that various add-on portions can be formed with different configurations.
Heat sinks are typically machined from metal stock, such as copper which has a high thermal conductivity. Evidently, many machine operations are required to form the fin layers, and much of the stock is removed in the machining operation. While the removed stock may be recovered, it represents initial waste.
The cost of machining is relatively high, and is exacerbated by the need to fabricate different heat sink structures for different devices. For example, one heat sink may have three fins, while another nearly identical heat sink requires five fins. With machining, there is little or no flexibility to accommodate differences from one heat sink to another.
U.S. Pat. No. 4,340,902 discloses a heat sinking arrangement wherein a cooling fin assembly is press-fitted around a cooling stud (see, e.g., FIG. 1 therein), and a similar arrangement wherein a cooling fin assembly is threaded onto a cooling stud (see, e.g., FIG. 2 therein). Also disclosed (see, e.g., FIGS. 3 and 4 therein) is an arrangement wherein a cooling stud is preformed with cooling fins in the form of a single unitary structure.